During this period Australia was still situated between 0° and 30°N. High global sea levels at this time saw large areas of epicontinental,
shallow, warm seas populated with rich assemblages of fauna, with abundant
corals. At the close of the period the global climate cooled, ushering in an ice
age that persisted until the Silurian. As more water was locked up in the vast
glaciers the sea levels fell around the world. Australia was not affected very
badly by the glaciation as it was near the equator at the time.

Australia was part of
Gondwana, and there were a number of
comparatively small blocks in the Northern Hemisphere that were evolving
into the landmasses. During the Ordovician, Gondwana swung away from the
equator, and by the end of the Period, the northern part of South
America was situated further into polar regions, but the position of
Australia was not changed much from that in the
Cambrian, being on the
opposite side of Gondwana to South America, the South American end having
swung around much more than the Australian end.

The climate appears to have been hot in the Early Ordovician, cooler
in the Middle Ordovician, hot again in the Late Ordovician, but cool
again near the end of the Period when polar ice caps were present that
lasted into the earliest
Silurian. The glacial period appears to have reached its maximum at
about 440 million years ago when the sea levels reached their
minimum. This glacial period has been called the "Early
Palaeozoic Icehouse", and studies by a
group of scientists at the University of Leicester suggests it may
have resulted from CO2 from the atmosphere being locked up and buried by
the dead plankton raining down to the seafloor where conditions at the
time were believed to have been anoxic. The plankton are believed to
have flourished in the warm, greenhouse type conditions of the earlier
part of the Ordovician and when they died they accumulated in large
quantities on the sea floor where the CO2 was not released by bacterial
decomposition. This sequestering of the CO2 eventually lead to a
depletion of the atmospheric CO2, hence the cooler conditions that led to the
ice age believed to have lasted for about 30 million years.

The rich fauna of the warm, shallow seas comprised members of some the
groups that were present in the Late Cambrian, with the addition of
groups such as the ancestral Horseshoe Crabs and Eurypterids that first
appear in the fossil record at this time. Crinoids first appear in the Early
Ordovician.

Australia

The Delamerian Orogeny that began in the Late Cambrian, continued
into the Ordovician, uplifting along a zone from western Tasmania to the
north-northwest through western Victoria to north-central-South
Australia. The high rate of erosion from this uplifted high country
indicates that the climate at the time was very wet, a high rainfall
being required to erode so rapidly.

The western part of the Amadeus Transverse Zone had been elevated
above sea level during the
Cambrian and the earliest part of the
Ordovician. In the Early Ordovician, it begun a long period of
subsidence that resulted in the formation of the
Canning Basin.
By the Middle Ordovician, the sea had covered this depression, linking
up with an embayment from the eastern continental margin to form the
Larapinta Sea that divided Australia into 2 separate parts separated by
a shallow sea. This sea eventually joined with a narrow channel that
extended along the continent's northwest margin, the first part of the
Westralian Depression. To the north of the Larapinta Sea the land was
low and featureless, while that to the south, made up of the southern
half of Western Australia and South Australia was rugged, high country.

Late in this period the first stirrings of the
Alice Springs Orogeny
began, and the epicontinental seas began to retreat from the land, the
Larapinta Sea shrinking as the sediments deposited in it were uplifted
and eroded. At this point the Amadeus Transverse Zone was no longer a
complete entity. Originally opening to the east in the Tasman Orogen but
now it opened to the west in the Westralian Depression.

Along the eastern margin, deep water sedimentation continued. In the
area of present-day eastern Australia,
volcanic activity was increasing and
becoming more widespread. Volcanic rocks were accumulating episodically,
the Lachlan Orogeny.

Because of the extensive areas of shallow-water environments during
the Ordovician there was a rich, diverse fauna. Filter-feeding organisms
flourished. There was a rapid burst of evolution among the brachiopods,
Bryozoans, Molluscs and Echinoderms, Graptolites being characteristic of
the period, their rapid change make them very useful in stratigraphy.
The Period has been divided into zones based on the changing types of
graptolites through time, allowing geographically separated strata to be
correlated.

A very large species of trilobite, a group that were very abundant at
the time, has been found that grew to about 50 cm in the Stairway
Sandstone that was deposited in the
Larapinta Sea. The trilobite
burrows, known as Cruziana, are often preserved as trace
fossils. Rusophycus are the scratch-marked mud piles they
produced when feeding. There were abundant predatory cephalopods, such
as straight-shelled nautiloids. The warm temperature of the water is
indicated by the presence of corals.

Fish, the first vertebrates, arose during the Ordovician,
Arandaspis
being among the first known craniates.
Arandaspis
and 3 other related species are found in the Stairway Sandstone.

The Receptaculitids are a group with a very complex structure
that arose in the Ordovician, surviving to the Devonian. They are now
believed to be coralline algae, previously being classified as problematica.

The Ordovician seas that covered much of Central Australia deposited
sediments containing some of the oldest fish fossils in the world. A variety of
other marine fossils are preserved in them - shelled nautiloids, molluscs,
trilobites.